Gus Chan, The Plain DealerDead rainbow darters discovered in the Rocky River in April.

CLEVELAND, Ohio -- Ohio and U.S. Environmental Protection Agency officials aren't ready to call off the dogs in determining who or what caused an estimated 28,613 fish to die in the East Branch of the Rocky River in late April.

"This is a continuing investigation, although EPA investigators from both the Ohio and federal offices have no new information to release," said Michael Tobin of the U.S. Attorney's Office in Cleveland. "These types of environmental cases can take quite a while to be resolved. We're leaving no stone unturned."

A toxic pollutant is the most likely cause of the fish kill, but toxins were not found in the watershed when fisheries biologists from the Ohio Division of Wildlife and the Ohio EPA sampled the East Branch.

If the incident appears to be something other than a complete accident, EPA investigators will be tight-lipped until they have the case resolved, said Mike Settles, a media relations representative for the Ohio EPA.

Most of the dead fish -- ranging from minnows to big rainbow and steelhead trout -- were found in a three-mile stretch of the East Branch of the Rocky River that runs through the Cleveland Metroparks from Bonnie Park Picnic Area in Strongsville to Wallace Lake in Berea.

The fish kill extended almost to the main branch of the Rocky River, where a few steelhead trout were found dead.

The East Branch is generally stocked weekly by the Cleveland Metroparks at that time of year with rainbow trout, making it a very popular fishing area. Since the spill, Wallace Lake has been the major destination for the farm-raised trout.